docs: local platform

lib/modules/platform/local/index.md

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+# Local
+
+The "local" platform exists to allow users to perform dry runs against the local file system, such as to test out new config.
+
+## Usage
+
+Run `renovate --platform=local` in the directory you want Renovate to run in.
+In this mode, Renovate will default to `dryRun=lookup`.
+No "repositories" arguments should be provided, as you cannot run against multiple directories or run in the non-working directory.
+
+It is possible to run on both a git and non-git directory.
+Config is optional - so it will run either with or without any "repo config" found.
+
+It does not do any "compare" or before and after analysis - if your purpose is to test a new config then you will need to manually compare.
+
+## Limitations
+
+- `local>` presets cannot be resolved. Normally these would point to the local platform such as GitHub, but in the case of running locally, it does not exist
+- `baseBranches` are ignored
+- Branch creation is not supported